A MOS solid-state imaging device is an image sensor that reads charge accumulated in a photodiode (photoelectric conversion section) by means of an amplifier circuit including a MOS transistor. In such a MOS solid-state imaging device, drive with low voltage and low power is allowed, and also pixel units each composed of a photodiode and the like arranged two-dimensionally, drive circuits (shift registers and the like) for driving MOS transistors in the pixel units and even a signal processing section for processing pixel signals detected in the pixel units can be formed on the same substrate. Hence, solid-state imaging devices of this type have been widely applied to image input devices and the like of mobile equipment.
In general, a photodiode constituting each pixel unit is electrically isolated with an element isolation section formed in a silicon substrate. A silicon oxide film and the like are formed on the surface of the photodiode, and this is likely to cause a dark current (charge generated in the dark time with no light illumination) due to a crystal defect occurring at the Si—SiO2 interface. Hence, for the purpose of reducing the dark current occurring near the surface of the photodiode, a buried photodiode structure has normally been adopted in which a high-density impurity layer is formed on the surface of the photodiode.
In the element isolation structure for electrically isolating photodiodes and the like, downsizing of pixel units can be achieved by adopting LOCOS (local oxidation of silicon) isolation in place of the conventional PN junction isolation. In the LOCOS isolation, however, a dark current is likely to occur due to a crystal defect occurring at the LOCOS interface. Hence, a high-density impurity layer is further formed at the LOCOS interface in the silicon substrate to thereby reduce the dark current occurring near the LOCOS interface.
The above configuration however has the following problem. The effective open area of a photodiode defined with the LOCOS isolation is reduced with the high-density impurity layer formed at the LOCOS interface, and as a result, the sensitivity of the photodiode degrades.
To solve the above problem, Patent Document 1 describes a solid-state imaging device adopting STI (shallow trench isolation) as the element isolation structure.
FIG. 9 is a cross-sectional view showing a configuration of a photodiode isolated with STI. As shown in FIG. 9, the photodiode is composed of a p−-type silicon layer 102 and an n-type silicon layer 103 formed in a silicon substrate 101, and is isolated with STI 104 formed in the silicon substrate 101. A p+-type surface layer 105 is formed on the surface of the n-type silicon layer 103, and a p+-type buried layer 106 is formed at the interface between the n-type silicon layer 103 and the STI 104, to thereby reduce a dark current occurring at these interfaces.
The STI 104 can be formed by forming a groove (concave portion) in the silicon substrate 101 and filling the groove with a silicon oxide film. The p+-type buried layer 106 can be formed after formation of the groove by introducing an impurity into a portion of the silicon substrate 101 located on each side of the groove by slant ion implantation. The p+-type buried layer 106 can therefore be thinned. Hence, the reduction in the effective open area of the photodiode can be minimized, and thus the sensitivity of the photodiode is suppressed from degrading along with the reduction in open area.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-318379    Patent Document 2: Japanese Laid-Open Patent Publication No. 2006-186204